1. Field of the Invention
This invention relates generally to the dewatering of paper webs in a papermaking process, and more particularly, to the use of capillary forces to remove water from unpressed wet webs without substantial overall compaction of the web during the papermaking process.
2. Brief Description of the Prior Art
U.S. Pat. No. 3,262,840 to Hervey relates to a method and system for removing liquids from fibrous articles such as paper and textiles using a porous polyamide body. The porous polyamide body is, for example, a resilient porous sintered nylon roll. In this method, a wet paper fiber web is passed through a series of pressure nips, each of which includes at least one porous nylon roll. Apparently, liquid is transferred from the wet paper fiber web into the porous nylon rolls by a combination of the pressure that is applied by the nip rolls, some degree of capillary action at the porous roll, and vacuum assistance. However, liquid transfer is substantially limited in this process because it must occur during the relatively short period of time in which the web passes between the nip and the opposed rolls. Hervey further discloses that the water taken in by the porous nylon roll is then either blown out of the pores by pressurizing a chamber within the roll or withdrawn from the pores by applying an external vacuum to the roll. This blowing out of the water from the pores also tends to clean the pores.
U.S. Pat. No. 4,556,450 to Chuang, et al., discloses a method and apparatus of removing liquid from webs through the use of capillary forces without compacting the web. The web passes over a peripheral segment of a rotating cylinder having a cover containing capillary-sized pores. The internal volume of the rotating cylinder is broken up into at least two and as many as six chambers, which are separated from each other by stationary parts and seals. At least one of the chambers has a vacuum induced therein to augment the capillary flow of water from the sheet. Another chamber includes a positive pressure to expel water from the pores outward of the cover after the sheet has been removed. Presumably, the pores are cleaned by this expulsion of water. All of the water taken from the sheet is held within or just under the pores and is expelled from the capillary cover at each revolution of the cylinder. A few cover materials are discussed, including a sinter-bonded Double Dutch Twill Weave as taught in U.S. Pat. No. 3,327,866 to Pall.
U.S. Pat. No. 4,357,758 to Lampinen teaches a method and apparatus for drying objects such as paper webs using a fine porous suction surface saturated with liquid and brought into hydraulic contact with a liquid that has been placed under reduced pressure with reference to the web being dried. The fine, porous liquid suction surface is located on the outside of a rotating drum and water is withdrawn from the drum apparently through the use of pumps which rotate with the drum. Lampinen does not seem to make any provision for cleaning the pores.
The prior art fails to teach the light knuckled pressing of the web against the capillary membrane to ensure hydraulic contact between the water contained in the web and the water in the pores of the capillary membrane without overall compaction of the web. This promotes greater and more rapid dewatering through the use of the capillary membrane. Further, lightly pressing the web against the capillary membrane with a knuckled surface is not taught in combination with a non-sectored capillary dewatering roll which is maintained at a single pressure throughout, that pressure approaching but not exceeding the effective capillary breakthrough pressure of the mean flow pore diameter of the capillary membrane. In addition, the prior art fails to disclose the washing and cleaning of the capillary membrane from the outside of the capillary dewatering roll to the inside thereby flushing any particulates trapped in the pores to the inside of the drum. This is possible because the drum is non-sectored and maintained at a single vacuum pressure, and further, because the capillary pores are substantially straight through, non-tortuous path pores.